slapping the sack

When someone with a tiny penis "try's" to masturbate. Their peter is so tiny that they end up just slapping their sack

Jessica: I caught Perry beating off in the bathroom yesterday.
Amanda: Trust me, his dick is so small he was just slapping the sack.

Slap Sack

A Slap Sack is a place where women keep their fond, sexual memories, so she can think about them while masterbating.

That was a good session babe! I'm gonna keep that on in my Slap Sack for later!